The carrier frequencies of the transmitters and receivers (transceivers) used in mobile wireless devices are synthesized from fixed, limited accuracy crystal oscillators. Thus in a pair of communicating wireless devices there may be a difference between the carrier frequencies of the respective transmitter and receiver and the difference, the carrier offset, shows up in the demodulated baseband signal. Although digital techniques can remove the effects of the carrier offset (CFO) by modifying the received signal stream as if it had been demodulated with the recovered transmitter carrier frequency; it is still called Automatic/Adaptive Frequency Control (AFC). The transformation caused by the carrier offset is deterministic but the signal stream also carries noise. Not to become the bottleneck, an AFC digital technique should tolerate noise at least as well as the rest of the baseband processing does.
The time it takes for the AFC to determine the carrier offset (“lock onto the transmitter carrier”) lengthens the required transmit frame preamble, thus shortens (through the increase in radio on-time) battery life, hence raises the operating costs of a wireless device. The amount of carrier offset the AFC can compensate determine the required oscillator crystal tolerance, hence influences the purchasing cost of a device.
The requirement on crystal accuracy becomes stricter with increasing payload bit rate. In multi-rate wireless transceivers the crystal accuracy must satisfy the requirement for the highest bit rate that may be counterproductive for the low cost applications at the lower bit rates. Such is the case with ieee802.15.4 wireless transceivers that also have a proprietary operating mode with 125 k-250 k-500 k-1M-2M bps bit rates or a subset thereof.